Sadly my FIL died a couple of months ago. Ds is 6 and saw FIL a couple of times a week. Since FIL died there's been a lot of talk about how he's watching us and he's still with us etc which isn't my personal belief but it's been helpful to them and it's what they believe.
However when we went round yesterday ds was being a bit of a pain. Nothing terrible, it was getting late and he was hungry so he was being whiny and grumpy. MIL said to him 'you'd better watch out, FIL will know if you're being naughty, he can see you all the time.'
I didn't say anything at the time but I wish I had now in case she says it again. I don't really want a dead person being held over ds as some sort of threat. To be fair ds hasn't said anything but he is the sort of child who says nothing for a while and then all of a sudden, sometimes weeks after an event, he'll suddenly say he's been worried by something.
Aibu to think it's not an appropriate thing to say to a just 6 year old? It would have worried me at the same age I think.